This pluggable interface allows you to validate the fields and their argument inputs before query execution.
You will be called with fields and their arguments expanded out ready for execution and you can check business logic
concerns like the lengths of input objects (eg an input string cant be longer than say 255 chars) or that the
input objects have a certain shape that is required for this query.
You are only called once with all the field information expanded out for you. This allows you to set up cross field business rules,
for example if field argument X has a value then field Y argument must also have a value say.